It may be necessary to generate seismic waves or other vibrations for several purposes, such as:
1. Structural testing by vibration; PA1 2. Communication by seismic signals; PA1 3. Seismic exploration.
The present invention, although capable of other applications, is particularly intended for and will be described as applied to seismic exploration.
Traditional seismic exploration techniques use an explosive energy source in order to map geological formations by recording seismic signals representing the reflected or refracted seismic energy. However, during the last ten years or so an alternative method which is known under the Registered Trade Mark "VIBROSEIS" has been established, in which the signals transmitted are swept frequency signals the duration of which may be many times greater than the total travel time of the signals when received. The resolving power of such a long duration signal is restored by cross-correlating the received signal with that which was transmitted.
Another technique which uses a signal of very long duration is described in U.S. Pat. No. 3,588,802. In this case the input signal may be of constant frequency, but it is necessary to be able to choose a frequency in any part of the seismic spectrum.
Both these extended signal techniques employ transducers (vibrators), which have the following advantage over the conventional use of explosive charges;-
1. The total energy in the signal may be very large but the peak power is relatively small so that there is negligible damage to road or other surfaces or structures where the signal is injected into the earth;
2. Precise control is possible over the phase and amplitude spectrum of the signal.
The second of these advantages has gained a greater significance with the increasing demand for higher resolution seismic surveys in the near-surface parts of the earth; for example, for more detailed mapping of fault zones in coal measures so that pit workings may be planned with greater efficiency.
Vibrators which are designed specifically for seismic exploration on land are usually hydraulic or electromagnetic transducers. The hydraulic transducer has the advantage that it will develop much more force than an electromagnetic transducer of the same size. However, it has the disadvantage that its upper frequency response is restricted to little over 100 Hz; this is mainly due to inertia in the mechanical parts of the hydraulic control system.
The electromagnetic transducer is not so restricted. Frequencies well above 1 kHz, which is beyond that which is useful in seismic exploration, are possible, but the electrical power amplifier necessary to generate the required force must be very large.
Another factor which determines the choice of transducer is the enormous rate of attenuation that occurs at the high frequency end of the spectrum as the seismic wave travels through the earth. In deep exploration for oil, where the wave may be expected to travel down to 30,000 ft. or more, the attenuation of frequency components above 80 Hz is so great that they are virtually lost in the ambient and system noises. Under these circumstances there is little point in using a signal above 100 Hz and, therefore, the hydraulic transducer is adequate. However, in near-surface work, where the depth of interest is between about 100 and 3,000 feet, it is possible to detect reflected signals with components above 200 Hz. Furthermore, the broader band signal that results from extending the frequency spectrum is essential if small details in the earth's layers are to be resolved.
Consequently there is renewed interest in the capability of the electromagnetic transducer, and it is clear that there would be an advantage if the efficiency could be improved so that smaller high-force units could be employed in near-surface exploration.
Existing seismic vibrators were adapted from electromagnetic "shakers" designed for environmental testing in industry. The unit resembles a gigantic drive unit similar to that used in a moving coil loud speaker, except that the "moving coil" is rigidly fixed to a base plate which rests on the ground, while the concentric magnet, which is supported by a spring, acts as the reaction mass. The unit is mounted on a vehicle which, in addition to containing the electrical power supply source, acts as a hold-down weight on the base plate so it does not leave the ground when the acceleration on the reaction mass exceeds that of gravity. Electrical power is converted into power in the form of seismic waves with an efficiency estimated to be about 1%.